This application claims the priority of German patent Application No. 199306648.6-32 filed Jul. 2, 1999, which is incorporated herein by reference.
The invention relates to an electrohydraulic pressure supply unit with a speed-controlled electric drive motor and a variable displacement pump that is coupled to and driven by the motor, and whose displacement volume can be changed with an adjustment member.
A plurality of uses has been discovered for the electrohydraulic pressure supply units. These units are used in electrohydraulic control devices, for the hydraulic lifting and lowering of loads, for the actuation of hydraulic servo components and, not last, in motor vehicle hydraulic systems. The electrohydraulic supply units, used for example in motor vehicle hydraulic systems, are used in power steerings and electrohydraulically actuated actuators. Until now, three different principles were used to control the volume flow delivered by the hydraulic pump.
According to one known control principle, the hydraulic output of a hydraulic pump driven by an electric motor is adjusted by controlling the speed of the electric motor. With systems of this type, the electric motor is operatively connected to the hydraulic pump and the volume flow delivered by the hydraulic pump depends on the speed of the electric drive motor. In addition to the drive speed, the hydraulic pump itself is not provided with any other systems for controlling the delivered volume flow. The electric drive motor therefore must meet extremely high requirements for the layout of these systems. The drive motor must be able to process high volume flows with small system pressures on the load side, as well as small volume flows with high system pressures on the load side. The delivered volume flow is proportional to the speed of the drive motor while the system pressure on the load-side is proportional to the torque generated by the motor. As a result, the drive motors of these electrohydraulic systems must be designed for high speed as well as a high torque, thus resulting in very involved and expensive electric drive motors.
For that reason, electrohydraulic systems have been proposed, for which the electric drive is operated with a constant speed, thus permitting an optimization of the electric motor toward a single speed. Owing to the fact that the electric motor is the most expensive component of an electrohydraulic system, cost advantages can be achieved if the electric motor is optimized toward a constant speed. The required volume flow of the hydraulic pump is adjusted by regulating a by-pass valve. This control principle presupposes that the pump constantly supplies at least the desired volume flow. Since the desired volume flow must also be made available if no hydraulic output is required, these systems have a relatively high, unused dissipation.
In particular in motor vehicles, the power that can be supplied by the vehicle electrical system for the electric drive of a pump is limited. That is the reason why mechanical drives are used for hydraulic pumps with a higher output, which are connected to the motor vehicle internal combustion engine, for example, by way of a belt drive or chain drive. The driving speed of the pump is thus coupled with the speed of the internal combustion engine and cannot be adjusted independently. The delivered volume flow of those systems is controlled through adjusting the motor displacement volume of the connected hydraulic pump. The system pressure controls the pump displacement volume in dependence on a reference pressure. These systems have the disadvantage that the delivered volume flow depends on the speed of the internal combustion engine. Since the necessary system pressure must also be made available at lower speeds of the internal combustion engine, pumps with relatively high maximum displacement volume must be used for these systems. However, the displacement volume should be adjustable to the smallest possible value for high speeds of the internal combustion engine because otherwise the system pressure would rise to unfavorably high values. Thus, the displacement volume of the pump must be adjustable over a relatively wide range for these systems, which results in large mechanical designs for the variable displacement pumps.
The object according to the invention is to provide an electrohydraulic pressure supply unit with variable displacement pump and controllable drive, which simultaneously permits the optimization of the electric drive as well as the optimization of the variable displacement pump with respect to minimizing the necessary component sizes.
This object according to the present invention by an electrohydraulic pressure supply unit which comprises: a speed-controlled electric drive motor; a variable displacement pump, for which the displacement volume (V) can be changed with an adjustment member, and having connecting lines and load connections; a device for determining the delivered volume flow (Qactual) of the pump; a regulating device for regulating the drive motor speed, with the input variables provided to the regulating device including at least the delivered volume flow (Qactual) and a desired volume flow (Qdesired); a pump adjustment unit for actuating the adjustment member of the variable displacement pump; a pressure gauge for determining the differential pressure (xcex94p) across the load connections, and, a control unit for actuating the pump adjustment device, with the control unit using at least one reference pressure (pref) and the differential pressure (xcex94p) as input variables.
The invention involves the joint operation of two energy converters that can be adjusted separately including an electric motor with speed control, which drives a variable displacement pump, and a variable displacement pump with variable displacement volume. The electric motor converts electrical output, consisting of voltage U and current I (loss-encumbered), into mechanical output, meaning into speed n and torque T. The pump converts (also loss-encumbered) this mechanical output of the motor into hydraulic output. The hydraulic output is determined by the pressure difference xcex94p across the pump and the volume flow Q. As a rule, the pressure difference xcex94p is impressed by the user onto the system. The volume flow Q is derived from the displacement volume and the motor speed n. The torque T, which must be generated by the motor, is derived from the displacement volume V and the pressure difference xcex94p. During the conversion from hydraulic output to mechanical output, the displacement volume V determines the distribution of the output to the speed n of the motor and the torque T. The current I flowing through the electric motor is primarily determined by the torque T while the voltage U is primarily influenced by the motor speed n. Structural size and electrical dissipation in the electric motor are primarily determined by the maximum torque T to be provided by the motor, meaning it is determined by the current I. The delivered volume flow for the electrohydraulic pressure supply is essentially adjusted via the speed adjustment of the controlled electric motor. The speed of the electric drive motor for the most part can be adjusted independently of the displacement volume of the variable displacement pump. For the most part independently means that only the respectively predetermined, desired volume flows and reference pressures that are required at the current time must be taken into account for adjusting the speed and the displacement volume. Within the range of these two specified system values, the drive motor speed can be adjusted completely independently of the displacement volume of the variable displacement pump. As a result, the displacement volume adjustment of the variable displacement pump can be pressure-controlled and the pump output, which is changed due to the displacement volume adjustment, can be compensated by the independent speed control of the electric motor. The volume flow can be detected either directly via a volume-flow meter or indirectly from the reaction of the machine tool, e.g., from the regulating distance of the connected actuator. The variable displacement pump can thus be operated as a device for influencing the torque, e.g., as a torque regulator for the electric motor.
The following advantages are primarily achieved with the invention:
The pressure difference xcex94p and the volume flow Q determine the hydraulic output. The volume flow Q is influenced by the speed n of the motor and the displacement volume V of the variable displacement pump. According to the invention, the combination of a speed-adjustable drive motor with a variable displacement pump with variable displacement volume V thus provides two degrees of freedom for influencing the hydraulic output. As a result, it is advantageously possible to adjust the power consumption of the electric motor to the respectively required hydraulic output. The destruction of delivered output in by-pass lines is thus advantageously avoided.
A further advantage of the combination according to the invention can be seen in that the necessary differential pressure xcex94p can be supplied for the most part independent of the torque T of the electric motor. The speed n together with the adjusted displacement volume V of the variable displacement pump also influences the differential pressure xcex94p. This fact is used advantageously for limiting the maximum torque T, which must be provided by the electric motor. It is even more advantageous if an electric motor with high speed and small torque is operated to supply the same output as an electric motor with low speed and high torque. With motors, the torque determines the structural size and thus the costs for the system. Using the variable displacement pump as a device for influencing the torque, e.g., as a torque regulator, advantageously permits limiting the torque, thus making it possible to have an optimum, low-loss operation with a smaller drive motor.
Another advantage of the combination according to the invention of a speed-controlled drive and a variable displacement pump with variable displacement volume is that an electrohydraulic pressure supply unit of this type can generate high pressure differences xcex94p for small volume flows Q, as well as high volume flows Q with small pressure differences xcex94p while the power consumption remains the same. Examples for such operating states, are on the one hand, highly dynamic adjustment movements and, on the other hand, holding functions under load. Operating states of this type occur in motor vehicles, for example, with activated running gears, electrohydraulic brakes and steering mechanisms. Electrohydraulic actuators for these systems can be operated more economically with the pressure supply unit according to the invention and can have a smaller, lighter and more cost-effective design.
Using the electrohydraulic pressure supply unit according to the invention in motor vehicles results in another advantage. The previously described option of limiting the torque has advantageous effects on the electronic control and power equipment. The torque T of the electric motor is essentially proportional to the current I. Thus, if the torque is limited, the current I is also limited. As a result, the electronic control and power equipment can also be designed for smaller current intensities. Owing to the fact that losses in the electronic control and power equipment are also primarily determined by the current intensity, the pressure supply unit according to the invention also permits an optimum design as well as the low-loss operation of the electronic control and power equipment.